Gas Carriers Basic Info
The two basic principles for understanding the way of Gas Products’ transportation are keeping pressure greater than the atmospheric pressure and/or keeping temperature below the ambient temperature.
Gas Carriers Categories
Gas carriers are divided into three main categories:
- Fully Pressurized
- Semi Pressurized and Semi Refrigerated
- Fully Refrigerated
Vessels fitted with a number of horizontal, cylindrical or spherical cargo tanks designed to work at a pressure of up to 18 kg/cm. Fully pressurized tankers represent a cost-effective, simply way of moving LPG normally to and from smaller gas terminals and in short distances.
Tanks material: Steel
Semi Refrigerated/ Semi Pressurized
By installing a reliquefaction plant, insulating the cargo tanks and making use of special steels, the thickness of the pressure vessels -and hence their weight- could be reduced. These carriers, incorporating tanks cylindrical, spherical or bi-lobe in shape, are able to load or discharge gas cargoes at both refrigerated and pressurized storage facilities. Vessels carry gases at temperature and pressure condition of up to -48°C and 8.5 kg/ cm² respectively.
Tanks material: Steels such as fully killed, finegrain, carbon-manganese steel, sometimes alloyed with 0,5% nickel.
Fully refrigerated carriers have prismatic-shaped cargo tanks fabricated from 3.5% nickel steel, allowing the carriage of cargoes at temperatures as low as –48°C and near atmospheric pressure. Prismatic tanks allow cargo carrying capacity to be maximized, thus makes the fully refrigerated tanker highly suitable for carrying large volumes of cargo such as LPG, ammonia and vinyl chloride over long distances.
Ethylene Carriers are used to transport ethylene fully-refrigerated at its atmospheric pressure boiling point of -104° C, and have greater flexibility as they are also capable of carrying most of the LPG cargoes. Tanks material: nickel-alloyed steels, stainless steels or aluminum.
Cargo Tank Types
The cargo tanks are categorized as following:
- Independent (Self-supporting)
- Membrane (Not Self-supporting)
Subject cargo tanks are divided in three subcategories:
- Type A: Apply to Fully Refrigerated vessels. Prismatic shape.
- Type B: Apply mainly to LNG Carriers. Usually spherical shape (Kvaerner Moss Type)
- Type C: Apply to Fully Pressurized and Semi Refrigerated Vessels. Cylindrical, spherical and bi-lobe shape.
Typical cargo tanks applied to conventional LNG Carriers.
Gas Carrier Types by Design
Gas Carriers are also classified on the basis of the hazard potential of the cargoes they carry and on the measures required to prevent escaping and/or contain such cargoes.
Type 1G designed to transport most hazardous products
Type 2G & 2GP - designed to transport less hazardous products
Type 3G - designed to transport the least hazardous products